The invention relates to a selective optical detector apparatus which is adapted for the reception of optical signals, when the spatially extending light bundle is parallel or can be formed into parallel and, when it is also substantially monochromatic. The detector should receive only the light which falls on a narrow wavelength range, and it should be insensitive to light which lies outside of such a range.
Worldwide considerable research efforts have been made in order to widen the operating frequency of locators, communication transmission devices and similar special measuring arrangements, into the optical region.
In the case of opto-electrical devices and measuring arrangements which are intended for the reception of spatially extending (cosmic, atmospheric and underwater signals) direct or reflected signals of slight intensity, the sensitivity of the receiver will become considerably reduced by the background light and interference light which fall onto the sensor surface. The background light and the interference light which fall within the operating angle of the receiver will result in a greatly reduced use of the electronic circuit elements within the receiver by reducing the otherwise available possibilities of sensitivity.
For this reason in the above-mentioned devices frequently the operating difficulties affecting the operating capacities during day and night, are separately listed. The function of a receiver of such opto-electronic devices resides conventionally in the reception of optical signals having a narrow spectrum. The sensing element usually is a photon multiplier or some type of light sensitive semiconductor device, which is adapted for the reception of a relatively wide spectrum and does not possess its own selectivity. In the hitherto known opto-electrical devices the influence of the background light and of the interference light has been attempted to be reduced by a narrowing of the operating angle (the angle of the field of vision) of the receiver and by employing a selective or color filter.
Such a solution has the following disadvantages: the effective angle of the receiver is now limited considerably on one hand due to the technical-technological limitation (stability of the aiming or sighting and of the fixation), and on the other hand, due to the nonuniform character of the transmission medium, the wandering of the light bundle in opto-electronic devices, the selection of the spectrum to be received, and the reduction of the background lights and of the interference lights will necessarily be performed by the use of filters. A relatively good selection can be accomplished by means of an interference filter having a small BW, the spectrum range of which has a value of 5-20 mm. Such interference filters are used in the receiver of many well-known opto-electronic devices. Their common disadvantages resides in that their use will lead to an additional damping. For such an application a number of examples can be found in the corresponding technical literature.
In the publication by B. G. King, P. J. Fitzgerald, and H. A. Steint entitled "An experimental study of atmospheric optical transmission" (The Bell System Technical Journal Vol. 62, No. 3. 1983) a narrow band filter is used in an optical receiver constructed as a communication experiment which will lead to a loss of 3 dB.
In addition to the filters a score of other solutions are used to reduce the influences of the background light and of the interference light, and for example, shielding cassettes are built for the optics and the operating angle of the receiver is limited. In the optical receiver according to M. J. Green entitled "Application of an Optical Data Link in an Airborne Scanning System" (Review of Scientific Instruments, Vol. 53, No. 8, 1982, pages 1278-1280), an infrared filter is used for filtering out the visible light, which, however, will not lead to any special result.
In the optical system according to G. Michael Lauham entitled "Air Force Lasercom Space Measurement Unit" (CH 1939-6/80, 1980, IEEE 27.2.1.-27.2.3) the receiver is provided with a narrow band filter (interference filter).
Thomas F. Wiener discloses in an article entitled "Strategic Laser Communications" (CH 1539-6/80 IEEE 27.4.1.-27.4.5) that the basic problem of outdoor receivers resides in the application of a corresponding filter and proposes as a solution the construction of a highly complicated crystal physical filter. The scheme of such a filter is described in the article entitled "EOTF Independently Controls Wavelength and Linewidth", which appeared in the September issue of the Electronic Design, 1979.
The use of interference filters and of resonance-narrow band filters which rest on a similar functional principle, will, however, run into a number of problems, of which the most significant ones are the following:
The interference filters are manufactured with a complicated technology during which extremely strict requirements with respect to the accuracy must be met, and, as a result, they are extremely expensive and their manufacturing in a mass production becomes very complicated;
From the selectivity viewpoint the interference filters offer a relative usefulness, however, they still create a considerable loss, which comes about by the strong damping of the useful signal, that is, in fixed construction into the device and in the absence of a background or interference light, the selectivity becomes superfluous but the sensitivity of the receiver will still be reduced (that is its operating range), since in this case without a filter the signal/noise ratio would not create any limitation on the sensitivity.
Building an interference filter into the receiver will complicate the optical system, since they exert their filtering affect mainly in the axial direction, onto the light bundle which is parallel with the optical axis of the system, a light bundle which will fall at a different angle with respect to the axis will create resonances on different wavelengths. A correction of an error can be had only by the addition of other optical elements, which will increase the number of the optical elements present. The associated increase in the number of the optical boundary surfaces will lead to a further loss, that is, to an increase of the damping;
The interference filters are manufatured for a fixed wavelength, they cannot be tuned, therefore the receiver cannot be set to a wavelength of the light which one could consider receiving. The wave pass BW therefore is defined by the results of the technology involved and, after the construction of the filter into the optical system, cannot be changed anymore. While the light source used as the transmitter will radiate wavelengths which scatter and, in the case of fast moving objects, such as cosmic objects, it will also become shifted.
Due to such limited possibilities of interference filters, other solutions were looked for, and for example, a filter has been developed by using dual-refracting crystals, which can be tuned with the help of acoustic waves and with an electrical field, which, however, turned out to be more complicated than the interference filters and its application has also led to a substantial amount of additional damping.
Due to the above-noted problems in simple structures of opto-electronic outdoor devices no interference filters or other optical filters were used which would have a high selectivity. Instead simple colored filters have been put in which were made from a material which has a substantially small selectivity and, which due to the absence of a high selectivity, cannot have a substantial increase in the ratio signal-noise and, at the same time they weaken the useful signal. Their most important advantage resides in that they offer to a certain extent a protection for the detector element against the thermal influences of the intensive background lights and interference lights. In a communication transmission device according to Sandor Takacs (Technical University, Budapest) entitled "Wideband communication transmission experiments in the optical range" (Hiradas-Technika, 1980, page 350) no protective means have been used for excluding the surrounding light. Also the receiver according to Swiss Pat. No. 625,923 does not offer a solution for the protection against the interfering influence of the background light.